1. Field of the Invention
The present disclosure relates to image sensor circuitry, and more particularly, to a buffered direct injection pixel circuit with a current mirror.
2. Description of the Related Art
In the field of optical sensors, the use of optical sensors to detect images in the infrared (IR) and other spectra has been known. Optical detectors such as InGaAs (Indium Gallium Arsenide) or other sensors in the IR band and other frequencies have been deployed in a focal plane with silicon readout circuits. Typically, the current output from the detector device has been delivered to an integration capacitor through an injection MOSFET which, in turn, is gated by a buffer to reduce the detector reverse-bias variations which arise due to photocurrent variations. This configuration is referred to as a buffered direct injection (BDI) readout circuit. A BDI circuit configuration according to known implementations is shown in FIG. 1.
The output from the BDI portion of those implementations can be delivered to a current mirror, as also shown in FIG. 1. The current mirror is used to either amplify or attenuate the input photocurrents in order to detect daylight-to-starlight scenes. By varying the voltage differences between BIAS and GAIN in FIG. 1, the ratio between Iout and Iin in FIG. 1 can vary by several orders of magnitude. The right-hand portion of the network is used to reset an integration capacitor used to control the timing of the light-gathering interval performed by the BDI or other readout circuits.
However, in BDI implementations such as that shown in FIG. 1, there are potential difficulties in the performance of the optical sensing and output generated by the BDI and current mirror circuits. Operating characteristics of the current mirror portion of the circuit, when driven by the buffered current generated by the BDI portion of the circuit, can cause a delay in the settling time of the scene being imaged, particularly when there is a large change in the amount of luminance in the scene.
Thus, there is a need for a pixel readout circuit with improved architecture having characteristics that are better optimized for use in an IR FPA.